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I'm getting the 78 back on the road and found a few things wrong. The PO had the wrong AFM installed so the pump ran whenever the key was on. I've corrected that by installing an AFM with the correct number of connections.


Fuel pressure goes to 40 psi when the fuel pump is forced to rwhen idling causes no change in FP. Clamping the return line causes 60 psi than settles to 55 psi. while the engine is off. Idle FP is only 30 while the FSM says it needs 36 and it drops to 0 when the engine is off. Applying vacuum to the pressure regulator


Is the fuel return line check valve in the fuel pressure regulator. Are there any other likely leak sources besides injectors and CSV? Those test good but the system doesn't hold pressure. I don't think the or CSV are leaking.
 

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fuel check valve is in the pump discharge.
The FPR has a soft seat that seals when the pressure drops and stops bypassing.

If you shut the car off and put a clamp on the return line under the FPR, and the pressure stays at 30 or whatever it was while running, then it's the FPR Seat that is weeping and losing pressure making for hard restarts.

If clamping the line doesn't stop it bleeding out to zero, then the fuel check valve in the discharge of the pump is leaking (or your internal bypass is not reseated after you lifted it at 60psi and showed it flowed deadheaded through the relief valve at 55 psi...)

If you need a pump discharge check valve, you can get them separately (not that go in the pump, they go in the line).

If it's the FPR seat, then you change the FPR.

Vice Grips, fuel pressure gauge, and a couple more starts will sort you out.
 

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I'm getting the 78 back on the road and found a few things wrong. The PO had the wrong AFM installed so the pump ran whenever the key was on. I've corrected that by installing an AFM with the correct number of connections.


Fuel pressure goes to 40 psi when the fuel pump is forced to rwhen idling causes no change in FP. Clamping the return line causes 60 psi than settles to 55 psi. while the engine is off. Idle FP is only 30 while the FSM says it needs 36 and it drops to 0 when the engine is off. Applying vacuum to the pressure regulator


Is the fuel return line check valve in the fuel pressure regulator. Are there any other likely leak sources besides injectors and CSV? Those test good but the system doesn't hold pressure. I don't think the or CSV are leaking.
36 psi engine off pump on (no vacuum). Force the pump to run by disconnecting the start solenoid signal wire (spade), turn key to start. read while holding key in start position (engine will not turn over).


about 30 psi engine on idling pump on (normal vacuum while idling 16-20 inHg)


Dropping to zero is no big deal as long as it takes some time, like on hour. Yes it should hold put it does not really matter. If it drops in seconds then you have a problem.
 
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